The invention relates to data transmission from a base station to a mobile telecommunications device using Code Division Multiple Access (CDMA) and, more particularly, to CDMA data transmission from a plurality of base stations operating in macrodiversity mode and connected to a packet switched network.
A fundamental principle of conventional CDMA systems is so-called macrodiversity, wherein a plurality of base transceiver stations (base stations) are communicating with one mobile station at the same time via the air interface. This technique, illustrated in FIG. 1, is used, for example, during conventional soft handover procedures when the mobile station is roaming from one base station (i.e. cell) to another. In downlink, the mobile telecommunications device (or mobile station) 11 receives data from all base stations BTS1 and BTS2 that are transmitting to the mobile station via the air interface 13. The mobile station then combines all data received to produce a combined result.
The CDMA frames transmitted by the base stations include conventional control information such as TPC (Transmit Power Control) commands so the mobile station can maintain proper output power, and pilot bits so the mobile station can maintain proper synchronization and perform coherent detection. The CDMA frames also include substantive information, that is, a message that the mobile station will combine with the corresponding message from the corresponding frame (or frames) from the other base station (or base stations). This CDMA macrodiversity technique is well known in the art.
The base stations will normally all transmit their data to the mobile station using CDMA in the air interface and, in the conventional macrodiversity technique, the base stations are synchronized to transmit their data over the air interface so the data from all base stations is received by the mobile station at the same time. If the base stations of FIG. 1 are connected as shown in FIG. 2 to receive packet data from a controlling node 21 in a packet switched network 23, it is possible that, due to the well-known nature of packet switching, one of the base stations will not have received its intended packet data at the time for the next CDMA data transmission over the air interface. It is also possible that one of the base stations will receive from the packet switched network packet data that has been corrupted in the transmission between the node 21 and the base station. Such data corruption is conventionally detected at the base station by using conventional (for example, Cyclic Redundancy Code (CRC)-based) error detection techniques.
In a prior art system, the base station transmits a dummy message (with TPC and pilot bits) in place of packet data that has not been received or has been corrupted, or alternatively, the base station transmits the corrupted message.
In conventional CDMA macrodiversity operation, the mobile station combines the substantive message information received from all of the base stations that are transmitting to the mobile station. When the same substantive message information is received from all of the base stations, the combined result produced by the mobile station is better than the result that would have occurred had less than all base stations transmitted that information to the mobile station. If some of the base stations send the same (correct) information, but one or more of the base stations sends information which differs from the correct information, then the combined result produced by the mobile station will be less reliable than if all base stations had sent the correct information.
It is therefore desirable for every base station to transmit the same, correct message information to the mobile station in CDMA macrodiversity mode.
According to the present invention, only those base stations that receive uncorrupted packet data will transmit message information, so every base station that transmits message information to the mobile station during CDMA macrodiversity mode will transmit the same, correct message information.